Time dependent fault detector

ABSTRACT

The RC time constant of a timer in a time dependent fault detector is automatically varied so that the timer may be used to monitor a plurality of mutually exclusive routines to which different maximum permissible periods of time are alloted. For example, the detector may be advantageously used to interrupt the operation of a xerographic copier if any one of a plurality of operating sequences of the copier is not completed on a timely basis.

United States Patent Mooney Dec. 23, 1975 TIME DEPENDENT FAULT DETECTOR[75] Inventor: Thomas J. Mooney, Fairport, N.Y. Prlmqry Exammer-HermanHohauser [73] Assignee: Xerox Corporation, Stamford, I

Comi- '57 ABSTRACT [22] Flled: 1974 The RC time constant of a timer in atime dependent [21] Appl. No.: 450,959 fault detector is'automaticallyvaried so that the timer may be used to monitor a plurality of mutuallyexclusive routines to which different maximum permissible 5 307/ periodsof time are alloted. For example, the detector n a I I I I v I e e u e I[58] new of Search f:i 1; of a xerographic copier if any one of aplurality of opcrating sequences of the copier is not completed on a[56] References Cited tlmely basls' UNITED STATES PATENTS 4 Claims, 3Drawing Figures 3,734,604 5/1973 Szostak et al 355/14 SCAN JAM JAM Sheet1 of 2 U.S. Patent Dec. 23, 1975 Sheet 2 of2 3,928,772

h k V I I I I I I III. \N H vm MM w l TIME DEPENDENT FAULT DETECTORBACKGROUND OF THE INVENTION This invention relates to failure detectorsand, more particularly, to time dependent failure detectors.

As is known, time dependent failure detection is a relativelystraightforward technique for monitoring any routine which normallyadvances from one identifiable point to another within a predeterminedamount of time. To carry it out, there typically is a timer having atime out period selected to equal the maximum time period required by aparticular routine under normal operating conditions, means fortriggering the timer into operation when the routine reaches the firstor reference point, and means for resetting the timer when the routinereaches the other or termination point. Thus, if the timer times out, itindicates that there has been a departure from nominal conditions or, inother words, that a failure has occured.

On occasion, several different time dependent fault detectors are usedin the same machine or process to monitor mutually exclusive routines,especially when those routines are alloted different amounts of time. Aswill be seen, this invention eliminates the need for resorting to thatwasteful practice.

SUMMARY OF THE INVENTION More particularly, one of the important objectsof the present invention is to provide a time dependent fault detectorwhich may be utilized to monitor a plurality of mutually exclusiveroutines, even if those routines are alloted different amounts of time.

Another object of this invention is to provide a method and means forautomatically adjusting the time out period of a time dependent faultdetector to match the maximum amount of time alloted to any one of aplurality of mutually exclusive routines.

A further object of this invention is to provide a time dependent faultdetector for automatically interrupting the operation of a xerographiccopier if any one of a plurality of mutually exclusive operatingsequences is not completed on a timely basis.

Still another object of the present invention is to provide a reliableand economical time dependent fault detector having the above-mentionedcharacteristics and capabilities.

In keeping with these and other objects, means are provided inaccordance with this invention for automatically adjusting the time outperiod of a timer in a time dependent fault detector. Mutually exclusiveroutines to which different maximum time periods are alloted can,therefore, be monitored by a single detector since the time out periodof the timer may be adjusted to match any one of them. Adjustment of thetime out period is conveniently accomplished by switching a transistorinto and out of conduction to thereby vary the RC time constant of thetimer.

BRIEF DESCRIPTION OF THE DRAWINGS Still further objects and advantagesof the present invention will become apparent when the followingdetailed description is read in conjunction with the attached drawings.in which:

FIG. 1 is a perspective view of a xerographic copier with which thepresent invention may be advantageously utilized;

FIG. 2 is a simplified schematic of the processor section of the copiershown in FIG. 1; and

FIG. 3 is a schematic of a time dependent fault detector constructed inaccordance with the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT While the inventionis described in some detail hereinafter with reference to a specificembodiment, it is to be understood that there is no intent to limit itto that embodiment. On the contrary, the intent is to cover allmodifications, alternatives, and equivalents falling within the spiritand scope of the invention as defined by the appended claims.

Turning now to the drawings, and at this point especially to FIGS. 1 and2, there is a xerographic copier 10 having a charging station 11, anexposure station 12, a development station 13, a transfer station 14 anda cleaning station 15 disposed about the periphery of a drum 16 which iscoated with a photoreceptor 17. In operation, an original document isplaced image side down on a transparent platen 18, a dial 19 is set tothe desired number of copies, and a print button 21 is depressed toinitiate the copying process.

Copying starts with the energization of a corona generator 22 at thecharging station 11 to uniformly charge the photoreceptor 17. Theoriginal document is flood illuminated by a suitable lamp (not shown)and is then scanned by a moving optics system 23 so that light reflectedfrom the document is focused on the photoreceptor 17 at the exposurestation 12. The reflected light discharges the photoreceptor 17 in animagewise configuration to thereby provide a latent electrostatic image.The latent image is developed at the development station 13 by theapplication of an electroscopic toner powder, and the developed image isthen transferred at the transfer station 14 to a copy sheet 24 which isfed from a cassette-type supply tray 25. Finally, to complete theprocess, the toner image is permanently affixed to the copy sheet 24 ata fuser station 26 and residual toner is removed from the photoreceptor17 at the cleaning station 15.

As will be appreciated, there are a number of routines which must besuccessfully completed on a timely basis to make a satisfactory copy,and some of those routines are mutually exclusive. For example, in theoptics system 23 there is a full rate scanning mirror 27 which ismounted on a carriage (not shown) to be reciprocatingly driven between ahome positioned beneath the left-hand margin of the platen 18 (as viewedin FIG. 2) and an end of scan position beneath the opposite orright-hand margin of the platen 18, as described in more detail in acopending and commonly assigned application of W. F. Hoppner, which wasfiled June 6, I973 under Ser. No. 367,996 on an Exposure Apparatus." Inthe ordinary course, the scanning mirror 27 returns to its home positionat the outset of each copy cycle and then moves towards its end of scanposition to scan the original document. Jam detection and copy quantitycontrol are, therefore, provided for this particular copier through theuse of a microswitch 28 which is positioned to be actuated when thescanning mirror 27 is in its home position and deactuated when themirror 27 is in any other position. Specifically, the jam detection andcopy quantity logic are dependent on the successful and repeatedactuation of the microswitch 28, as more fully explained in a copendingand commonly assigned application of L. J. Fantozzi, which was filedApr. 6, 1973 under Ser. No. 348,828 for Control Circuitry for TroubleDetection and Recovery System in a Copier/Duplicator." Accordingly, ifthere is a failure within the drive mechanism (not shown) for the opticssystem 23 or of the microswitch 28, jam detection and copy quantitycontrol are lost, thereby creating the risk of a run away machinecondition and of smoke and odor from undetected jams.

Analysis of this particular copier reveals that a failure of the drivesfor the optics 23 or of the microswitch 28 is indicated whenever thatswitch (1) is not actuated within six seconds or so after the printbutton 21 is depressed or (2) remains actuated during the copying cyclefor more than about six tenths of a second. Time dependent failuredetection may, therefore, be advantageously utilized. But there are apair of mutually exclusive routines to be monitored, and those routinesare alloted different amounts of time.

Referring to FIG. 3, there is a time dependent fault detector 31 whichis capable of monitoring mutually exclusive routines, even if they arealloted different amounts of time. The capabilities of the detector 31have been synergistically extended in accordance with this invention byincluding a timer 32 having an automatically adjustable time out period.To that end, as shown, the timer 32 comprises a three terminal thyristor33 which is switched from one state of conduction to the other when atiming capacitor 34 charges to a predetermined voltage level. Thecharging time con stant for the capacitor 34 is adjusted under thecontrol of a transistor 35 to thereby match the time out period of thetimer 32 to the particular routine being monitored, and the timer 32 istriggered into operation and reset under the control of an inverter 36.

More particularly, in the illustrated embodiment, current is drawn froma dc. power supply through a resistor 37 and the parallel combination ofcollectoremitter circuit of the transistor 35 and another resistor 38.The output of the inverter 36 is connected in parallel with the seriescombination of the timing capacitor 34 and a further resistor 39 in theground return path for that current flow. Thus, the current drawn fromthe dc. power supply is returned to ground through the timing capacitor34 and it series resistor 39 or through the output of the inverter 36depending on the logic level of the input signal applied to the input ofthe inverter 36. Specifically, if that signal is at a high (1) logiclevel, the timer 32 is triggered into operation because the current isrouted through the timing capacitor 34 to charge it toward the criticalvoltage level. If, on the other hand, a low logic level signal isapplied to the inverter 36, its output provides a relatively lowimpedance shunt path to ground for the current drawn from the dc. powersupply and for any discharge current drawn by the capacitor 34.

Now, when the timer 32 is triggered into operation, its time out perioddepends on the state of conduction of the transistor 35. If thattransistor is in a nonconductive state, the RC charging time constantfor the timing capacitor 34 is relatively long and the time out periodof the timer 32 is, therefore, relatively long. But, if the transistor35 is in a conductive state, the resistor 37 is effectively bypassedand, consequently, the charging time constant for the capacitor 34 isreduced to thereby reduce the time out period of the timer 32.

To ensure that the timer 32 is relatively insensitive to ordinaryfluctuations in the output current of the dc. supply source, thethyristor 33 is a programmable unijunction transistor (PUT). As shown,the PUT 33 has its anode-cathode circuit coupled across the timingcapacitor 34 and the resistor 39 and its gate coupled to a junctionbetween a pair of voltage dividing resistors 41 and 42 which, in turn,are connected across the dc. supply source. As a result, there is agate-cathode bias voltage which holds the PUT 33 in a non-conductivestate until the timer 32 times out. When that occurs, however, the PUT33 switches into conduction because there then is sufficient voltageacross the timing capacitor 34 to cause the anode-cathode voltage on thePUT 33 to exceed its gate-cathode bias voltage. In other words, time outof the timer 32 is marked by a significant drop in the gate-cathodevoltage of the PUT 33.

The input and output interfaces 43 and 44, respectively, for thedetector 31 are more or less tailored to its particular application tothe copier 10 (FIGS. 1 and 2). That is, they are selected, together withthe values of the capacitor 34 and of the resistors 37-39 and 41-42, tointerrupt the operation of the copier 10 if the microswitch 28 (l) isnot actuated within six seconds or so after the print button 21 isdepressed or (2) remains actuated for more than about six tenths of asecond at any time during a copying cycle. Indeed, to accomplish that,the input interface relies on certain signals that are supplied by thecopier 10 viz., (a) an initializing signal (INIT) which drops from ahigh l) logic level to a low (0) logic level approximately 20milliseconds after the print button 21 is depressed, (b) a scan signal(SCAN) which is at a high (1) or a low (0) logic level depending onwhether the microswitch 28 is actuated or not, and (c) a run signal(INIT BJAMF) which is herein assumed to simply be the complement of theinitializing signal (INIT). For a description of the provision made inthe copier 10 to supply those signals, reference may be had to theaforementioned application Ser. No. 348,828.

Inasmuch as the interfaces 43 and 44 form no part of the presentinvention, other than to mate the detector 31 with the copier 10, thereis no reason to burden this disclosure with a detailed discussion ofthem. Indeed, the drawings provide sufficient detail to make theiroperation self-evident.

CONCLUSION In view of the foregoing, it will now be appreciated that thepresent invention provides a time dependent fault detector suitable formonitoring a plurality of mutually exclusive routines, even if differentamounts of time are alloted to those routines.

What is claimed is:

1. In combination with a xerographic copier having a plurality ofmutually exclusive routines each alloted a different amount of time, atime dependent fault detector for interrupting the operation of saidcopier whenever any one of said routines fails to be completed withinthe time alloted thereto; said detector comprising a timing capacitor, aresistive path for supplying charging current for said capacitor, ashunt path coupled in parallel with said capacitor for bypassing saidcharging current around said capacitor until one of said routines isinitiated, means responsive to the initiation of any one of saidroutines for disabling said shunt path to thereby trigger said timerinto operation, and means for selectively bypassing a portion of theresistance in 6 circuit; and further including means for applying a biasvoltage to said gatecathode circuit, whereby said programmahleunijunction transistor switches from a nonconductive state to aconductive state when said timer times out.

4. The combination of claim 3 wherein said bypass means includes atransistor having a collector-emitter circuit connected in parallel withsaid resistance, whereby the time out period of said timer is adjustedby switching said transistor into and out of conduction.

1. In combination with a xerographic copier having a plurality ofmutually exclusive routines each alloted a different amount of time, atime dependent fault detector for interrupting the operation of saidcopier whenever any one of said routines fails to be completed withinthe time alloted thereto; said detector comprising a timing capacitor, aresistive path for supplying charging current for said capacitor, ashunt path coupled in parallel with said capacitor for bypassing saidcharging current around said capacitor until one of said routines isinitiated, means responsive to the initiation of any one of saidroutines for disabling said shunt path to thereby trigger said timerinto operation, and means for selectively bypassing a portion of theresistance in said path to thereby adjust the time out period of saidtimer to match the time alloted to said one routine.
 2. The combinationof claim 1 wherein said timer further includes a thyristor whichswitches from one state to another when said capacitor charges to apredetermined voltage level, thereby marking time out of said timer. 3.The combination of claim 2 wherein said thyristor is a programmableunijunction transistor having an anodecathode circuit and a gate-cathodecircuit, and said capacitor is coupled across said anode-cathodecircuit; and further including means for applying a bias voltage to saidgatecathode circuit, whereby said programmable unijunction transistorswitches from a non-conductive state to a conductive state when saidtimer times out.
 4. The combination of claim 3 wherein said bypass meansincludes a transistor having a collector-emitter circuit connected inparallel with said resistance, whereby the time out period of said timeris adjusted by switching said transistor into and out of conduction.